A separation method using a membrane has lots of advantages over the method based on heating or phase-changing. Among the advantages is high reliability of water treatment since the water purity required can be easily and stably satisfied by adjusting the size of the pores of a membrane. Furthermore, since the separation method using a membrane does not require a heating process, a membrane can be used with microorganism which is useful for separation process but may be adversely affected by heat.
Among the membrane employing separation methods is a method using a hollow fiber membrane module which comprises a bundle of hollow fiber membranes. Conventionally, the hollow fiber membrane module has been widely used in a micro-filtration field for producing axenic water, drinking water, super pure water, and so on. Recently, however, the application of the hollow fiber membrane module is being expanded to include sewage and waste water treatment, solid-liquid separation in a septic tank, removal of suspended solid (SS) from industrial wastewater, filtration of river, filtration of industrial water, and filtration of swimming pool water.
One kind of the hollow fiber membrane modules is a submerged-type hollow fiber membrane module which is submerged into a water tank filled with fluid to be treated. Negative pressure is applied to the inside of the hollow fiber membranes, whereby only fluid passes through the wall of each membrane and solid elements such as impurities and sludge are rejected and accumulate in the tank. When used for separation, the plural submerged-type hollow fiber membrane modules are installed in a frame structure. A submerged-type hollow fiber membrane module is advantageous in that the manufacturing cost is relatively low and that the installation and maintenance cost may be reduced since a facility for circulating fluid is not required.
However, when a submerged-type hollow fiber membrane module is used to treat wastewater, the solids in the wastewater fouls the membranes causing their permeability to be declined as the water treatment is processed. Thus, while the water treatment is carried out by the hollow fiber membrane module in a water-treatment tank, a maintenance cleaning has to be performed for stably maintaining the high permeability of the membranes. A typical method for the maintenance cleaning is an aeration method. In the aeration method, air is jetted from an aeration pipe positioned under the hollow fiber membrane module during the water treatment, thereby generating rising air bubbles. Thus, foreign materials are removed from the membrane surface owing to the rising air bubbles themselves and the rising water flow caused by the rising air bubble.
Since the maintenance cleaning based on the aeration method uses the rising air bubbles, it is important to maximize the friction between the rising air bubbles and the hollow fiber membrane so as to facilitate the high cleaning efficiency. In this respect, arranging the bundle of hollow fiber membranes to be vertical to the aeration pipe is preferred than arranging them to be parallel to the aeration pipe. Accordingly, a vertical-type hollow fiber membrane module is preferred than a horizontal-type hollow fiber membrane module.
A typical vertical-type hollow fiber membrane module includes an upper header, a lower header, and a bundle of hollow fiber membranes vertically arranged between the upper and lower headers.
Both ends of each of the bundle of the hollow fiber membranes are potted to the upper and lower headers respectively with an adhesive such as polyurethane. For making the potting procedure easier, a fixing rod may be temporarily used so as to stably maintain a predetermined interval between the upper header and the lower header. However, when the fabricated hollow fiber membrane module is installed to the frame structure, the fixing rod is removed preferably. This is because the unchangeable interval between the upper header and the lower header due to the fixing rod does not allow a slightest error in the coupling structure of the frame when the module is installed to it, thereby causing difficulties in installing the hollow fiber membrane module to the frame structure.
In this reason, most of the hollow fiber membrane modules have the upper and lower headers connected with each other only through the bundle of hollow fiber membranes, and the interval between the upper and lower headers is variable. In order to install this type of the hollow fiber membrane module to the frame structure, it is necessary to install the upper and lower headers independently to the frame structure at the same time or approximately at the same time. In case of a small-sized and light-weight hollow fiber membrane module, a man can install it without difficulties. However, with the current trend toward the large-sized hollow fiber membrane module for more water treatment capacity, there is limitations in installing a large-sized hollow fiber membrane to a frame structure only with the human power.